docs: def _receive(self):
      """Wait for a message from the worker process and return its payload.
  
      Raises:
        Exception: An exception was raised inside the worker process.
        KeyError: The received message is of an unknown type.
  
      Returns:
        Payload object of the message.
      """
      message, payload = self._conn.recv()
      # Re-raise exceptions in the main process.
      if message == self._EXCEPTION:
        stacktrace = payload
        raise Exception(stacktrace)
      if message == self._RESULT:
        return payload
      raise KeyError('Received message of unexpected type {}'.format(message))